How can i prove to my husband that taking ritalin (methylphenidate) is making his behaviour worse since starting on it when he thinks he is doing better?

Tricky!: It sounds as if he has been diagnosed with ADD or ADHD and prescribed medicine for treatment? Your husband maybe noting the good effects for the medicine, better progress at work, less distractibility, ability to complete projects, whereas you are noting a side effect of the ritalin, (methylphenidate) which has a narrow therapeutic range. A lower dose may provide the good effects with less side effects.

Document specific: behaviors & the time each occurs. Ask that he & his doctor review your list to determine any correlation with the waxing & waning of short-acting Ritalin's effect. Multiple daily doses, each given 1/2 hr. before the prior dose wears off, are needed to achieve consistent blood level & clinical effect. Your observations, not judgements, can help them in their decision-making about medications.

Accompany him to do: You should accompany your husband to a visit to his prescriber and have a discussion then. If that is not helpful consider couple's counseling.